Wavelength-variable short pulse generating device and method

ABSTRACT

An apparatus and method for generating a wavelength-tunable short pulse that can generate wavelength-tunable short pulsed light in the visible-light wavelength band are provided. When ultrashort pulsed light is introduced into an optical fiber ( 3 ), a wavelength-tunable ultrashort soliton pulse is generated by a nonlinear optical effect through the soliton effect and Raman scattering. When the soliton pulse has a short duration and high peak intensity, a third harmonic having one-third of the wavelength of the soliton pulsed light is generated by a third nonlinear optical effect. This third harmonic has a short wavelength in the visible light band.

TECHNICAL FIELD

The present invention relates to an apparatus and a method forgenerating a wavelength-tunable short pulse. More particularly, thepresent invention relates to a technique to generate wavelength-tunableshort pulsed light by a nonlinear optical effect in an optical fiber andthe third harmonic of the generated wavelength-tunable short pulsedlight, thereby obtaining wavelength-tunable short pulsed light in ashort-wavelength band. The apparatus and method of the present inventionfind particular application in optoelectronics, optical measurement, andmeasurement of spectra and biological material.

BACKGROUND ART

The present inventors have developed a technique to generatewavelength-tunable short pulsed light through a combination of anoptical fiber and an ultrashort optical-pulse source (see JapaneseUnexamined Patent Application Publication No. 2000-105394).

DISCLOSURE OF INVENTION

With the aforementioned technique, however, pulsed light with a shortwavelength in the visible-light wavelength band cannot be generated.

It is an object of the present invention to provide an apparatus andmethod for generating a wavelength-tunable short pulse that can generatewavelength-tunable short pulsed light in the visible-light wavelengthband.

To achieve the aforementioned object, the present invention provides thefollowing:

(1) An apparatus for generating a wavelength-tunable short pulseincludes: an ultra-short optical pulse source; an optical-propertyregulator for regulating the properties of an output from theultra-short optical pulse source; and an optical fiber for receiving theoutput from the optical-property regulator, the optical fiber generatingwavelength-tunable ultrashort pulsed light by a nonlinear optical effectthrough the soliton effect and Raman scattering and generating a thirdharmonic of the wavelength-tunable ultrashort pulsed light by athird-order nonlinear optical effect.(2) In the apparatus for generating a wavelength-tunable short pulse asset forth in (1), the optical-property regulator is a light-intensityregulator.(3) In the apparatus for generating a wavelength-tunable short pulse asset forth in (1) or (2), the wavelength of the pulsed light is alteredby changing the intensity of light input to the optical fiber by thelight-intensity regulator, thereby controlling the wavelength of thethird harmonic.(4) In the apparatus for generating a wavelength-tunable short pulse asset forth in (1), (2), or (3), the wavelength of the pulsed light isaltered by changing the length of the optical fiber, thereby controllingthe wavelength of the third harmonic.(5) A method for generating a wavelength-tunable short pulse includesthe steps of: receiving an output from an ultra-short optical pulsesource at an optical fiber, the output having passed through anoptical-property regulator; generating wavelength-tunable ultrashortpulsed light by a nonlinear optical effect through the soliton effectand Raman scattering in the optical fiber; and generating a thirdharmonic of the wavelength-tunable ultrashort pulsed light by athird-order nonlinear optical effect in the optical fiber.(6) In the method for generating a wavelength-tunable short pulse as setforth in (5), the optical-property regulator is a light-intensityregulator.(7) In the method for generating a wavelength-tunable short pulse as setforth in (5) or (6), the wavelength of the pulsed light is altered bychanging the intensity of light input to the optical fiber by thelight-intensity regulator, thereby controlling the wavelength of thethird harmonic.(8) In the method for generating a wavelength-tunable short pulse as setforth in (5), (6), or (7), the wavelength of the pulsed light is alteredby changing the length of the optical fiber, thereby controlling thewavelength of the third harmonic.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a conceptual view of an apparatus for generating awavelength-tunable short pulse according to the present invention.

FIG. 2 is a block diagram of an apparatus for generating awavelength-tunable ultrashort pulse according to a first embodiment ofthe present invention.

FIG. 3 is a block diagram of an apparatus for generating awavelength-tunable ultrashort pulse according to a second embodiment ofthe present invention.

FIG. 4 shows a soliton spectrum according to an example of the presentinvention.

FIG. 5 shows a waveform of an autocorrelation function for awavelength-tunable soliton pulse generated in the apparatus according tothe example of the present invention.

FIG. 6 shows a spectrum of the third harmonic generated in the apparatusaccording to the example of the present invention.

FIG. 7 is a graph showing dependency of the shift in the wavelength ofthe third harmonic pulse generated in the example of the presentinvention upon the length of the optical fiber.

FIG. 8 is a graph showing dependency of the wavelength of the thirdharmonic pulse generated in the example of the present invention uponthe intensity of light input to the optical fiber.

FIG. 9 shows a photograph (substituted for drawing) of the thirdharmonic scattered off the surface of the optical fiber of the exampleaccording to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will now be described in detail.

FIG. 1 is a conceptual view of an apparatus for generating awavelength-tunable short pulse according to the present invention.

In FIG. 1, 1 denotes an ultra-short optical pulse source, 2 denotes alight-intensity regulator, which is connected to the ultra-short opticalpulse source 1, and 3 denotes an optical fiber, which is connected tothe light-intensity regulator 2.

When an output from the ultra-short optical pulse source 1 is input tothe optical fiber 3 through the light-intensity regulator 2,wavelength-tunable ultrashort pulsed light is generated by the nonlinearoptical effects in the optical fiber 3, that is, the soliton effect andRaman scattering effect, and, in turn, the third harmonic of thewavelength-tunable ultrashort pulsed light is generated by thethird-order nonlinear optical effect in the optical fiber 3.

The wavelength of the wavelength-tunable ultrashort pulsed light can bealtered by changing the intensity of the light entering the opticalfiber 3 by the light-intensity regulator 2, which, in turn, controls thewavelength of the third harmonic.

Alternatively, the wavelength of the wavelength-tunable ultrashortpulsed light can be altered by changing the length of the optical fiber3 to control the wavelength of the third harmonic.

Now, generation of the third harmonic will be described in detail. Asdescribed above, input of the ultrashort pulsed light to the opticalfiber 3 generates a wavelength-tunable ultrashort soliton pulse by thesoliton effect and Raman scattering. When the wavelength-tunableultrashort soliton pulse has short duration and high peak intensity, thethird harmonic with one-third of the wavelength of the pulsed light isgenerated by the third-order nonlinear effect. This third harmonic has ashort wavelength in the visible light band.

FIG. 2 is a block diagram of an apparatus for generating awavelength-tunable ultrashort pulse according to a first embodiment ofthe present invention.

In FIG. 2, 11 denotes a femtosecond fiber laser serving as theultrashort optical pulse source, 12 denotes a light-intensity regulator,which is connected to the femtosecond fiber laser 11, 13 denotes a lightchopper, which is connected to the light-intensity regulator 12, 14denotes a polarization maintaining fiber, which is connected to thelight chopper 13, 15 denotes a spectroscope, which is connected to thepolarization maintaining fiber 14, 16 denotes a photomultiplier, whichis connected to the spectroscope 15, 17 denotes a lock-in amplifier 17,which is connected to the light chopper 13 and the photomultiplier 16,and 18 denotes a personal computer (PC), which is connected to thespectroscope 15 and the lock-in amplifier 17.

The femtosecond fiber laser (ultrashort optical pulse source) 11constitutes a pump light source, and the light-intensity regulator 12 iscomposed of a waveplate and an optical splitting unit for splittingpolarized light. The thin polarization maintaining fiber 14 has a corediameter of 6 μm.

An output from the light-intensity regulator 12 passes through the lightchopper 13 to propagate through the polarization maintaining fiber 14.The output from the polarization maintaining fiber 14 passes through thespectroscope 15 to be detected by the photomultiplier 16. Then, theoutput from the photomultiplier 16 is amplified through the lock-inamplifier 17. A signal from the light chopper 13 is input to the lock-inamplifier 17. The lock-in amplifier 17 and the spectroscope 15 areconnected to the PC 18 and thus function as an automatic measuringsystem.

FIG. 3 is a block diagram of an apparatus for generating awavelength-tunable ultrashort pulse according to a second embodiment ofthe present invention.

In FIG. 3, 21 denotes a femtosecond fiber laser serving as an ultrashortoptical pulse source, 22 denotes a light-intensity regulator, which isconnected to the femtosecond fiber laser 21, 23 denotes a waveplate,which is connected to the light-intensity regulator 22, 24 denotes anoptical fiber, which is connected to the waveplate 23 and has a shortlength and a small diameter, 25 denotes a wavelength filter, 26 denotesa spectroscope, and 27 denotes a photomultiplier.

The second embodiment accomplishes the following:

1. Since the waveplate 23 is disposed close to the end of the opticalfiber 24 from which light enters, the direction of the polarized lightis adjusted; that is, the polarized light becomes parallel to the axisof birefringence.

2. Since a polarization maintaining fiber constitutes the optical fiber,the light is kept linearly polarized while propagating through theoptical fiber.

Furthermore, since the optical fiber 24 has a short length (10 m orless) and a small diameter (6 μm or less), anomalous dispersion occurstherein. Accordingly, the pulse is compressed in the optical fiber 24,which, in turn, increases the peak light intensity.

A combination of 1 and 2 achieves the third-order nonlinear effect.

Furthermore, since the wavelength filter 25 is disposed close to theother end of the optical fiber 24 from which light is emitted, only thethird harmonic passes through the wavelength filter 25.

FIG. 4 shows a soliton spectrum according to an example of the presentinvention. In the spectrum, the abscissa represents wavelengths (nm),whereas the ordinate represents spectrum intensity (arbitrary unit).

FIG. 4 shows a spectrum of pump light and wavelength-tunable solitonpulses obtained from outputs using a 5-m optical fiber. A Sech²-shapewavelength-tunable soliton pulse was generated on the longer wavelengthside of the pump light. The intensity of the light input to the fiberwas 40 mW.

FIG. 5 shows a waveform of an autocorrelation function for thewavelength-tunable soliton pulse according to the example of the presentinvention. In the graph, the abscissa represents time (ps), whereas theordinate represents intensity (arbitrary unit).

There were no pedestal components and a neat waveform corresponding tothe Sech² pulse shape was observed. The duration for the waveform was 74fs.

FIG. 6 shows a spectrum of the third harmonic generated in the exampleof the present invention. The abscissa represents wavelengths (nm),whereas the ordinate represents spectrum intensity (arbitrary unit).

As shown in FIG. 6, a pulse spectrum was generated at one-third of thewavelength of the soliton pulse shown in FIG. 4.

FIG. 7 is a graph showing dependency of the shift in the wavelength ofthe third harmonic pulse generated in the example of the presentinvention upon the length of the optical fiber. In the graph, theabscissa represents lengths of the optical fiber (m), whereas theordinate represents the wavelength of the third harmonic (nm) generatedby third harmonic generation (THG).

As apparent from FIG. 7, as the length of the optical fiber increased,the wavelength of the wavelength-tunable soliton pulse shifted towardsthe longer wavelength side in accordance with the shift of solitonself-frequency. Consequently, the wavelength of the generated thirdharmonic pulse shifted towards the longer wavelength side. Thewavelength of the third harmonic pulse was one-third of the wavelengthof the wavelength-tunable soliton pulse.

FIG. 8 is a graph showing dependency of the wavelength of the thirdharmonic pulse generated in the example of the present invention uponthe intensity of light input to the optical fiber. In FIG. 8, theabscissa represents the intensity of light input to the optical fiber(mW), whereas the ordinate represents the wavelength of the thirdharmonic pulse (nm) generated through THG.

As apparent from FIG. 8, as the intensity of light input to the opticalfiber increased, the wavelength of the wavelength-tunable soliton pulselinearly shifted towards the longer wavelength side. Accordingly, thewavelength of the generated third harmonic linearly shifted towards thelonger wavelength side.

FIG. 9 shows a photograph (substituted for drawing) of the thirdharmonic scattered off the surface of the optical fiber of the exampleaccording to the present invention.

As shown in the drawing, as the pulsed light propagated through theoptical fiber, the wavelength of the soliton pulse monotonically shiftedtoward the longer wavelength side. Consequently, the generated thirdharmonic shifted in the order of green, yellow, orange, and red. Byobserving the wavelength or color of the third harmonic, the shift inthe wavelength of the pulsed light propagating through the optical fibercould be determined.

With the above-described structure, a third harmonic with color that isintense enough to be perceived is generated so that the change inwavelength in accordance with the propagation of the pulsed lightthrough the optical fiber can be visually observed. Alternatively, thechange in wavelength can be observed by setting a frequency meter in theoptical fiber.

The aforementioned generation of the colored third harmonic suggeststhat, in the future, three primary colors can be output with a singleoptical fiber, which offers great advantages.

In the above-embodiment, the light source and the light-intensityregulator are illustrated in different blocks. Alternatively, a unithaving functions of the light source and the light-intensity regulatormay constitute a single block.

The present invention is not limited to the above embodiments but mayrather be modified within the scope of the present invention. Thepresent invention encompasses these modifications.

As has been described above, the present invention achieves thefollowing advantages.

(A) Wavelength-tunable short pulsed light can be generated, the lightcontinuously shifting in the visible wavelength band relative to theintensity of light input to the optical fiber.

(B) A third-harmonic pulse can be generated simply by inputting pulsedlight to the optical fiber.

(C) The shift of the wavelength-tunable soliton pulse in accordance withthe propagation of the light through the optical fiber is readilyobserved.

INDUSTRIAL APPLICABILITY

The apparatus and method for generating a wavelength-tunable short pulseof the present invention are particularly suitable for optoelectronics,optical measurement, and measurement of spectra and biological material.

1. An apparatus for generating a wavelength-tunable short pulse,comprising: (a) an ultra-short optical pulse source; (b) anoptical-property regulator for regulating the properties of an outputfrom the ultra-short optical pulse source; and (c) an optical fiber forreceiving the output from the optical-property regulator, the opticalfiber generating wavelength-tunable ultrashort pulsed light by anonlinear optical effect through the soliton effect and Raman scatteringand generating a third harmonic of the wavelength-tunable ultrashortpulsed light by a third-order nonlinear optical effect.
 2. The apparatusfor generating a wavelength-tunable short pulse according to claim 1,wherein the optical-property regulator is a light-intensity regulator.3. The apparatus for generating a wavelength-tunable short pulseaccording to claim 1 or claim 2, wherein the wavelength of the pulsedlight is altered by changing the intensity of light input to the opticalfiber by the light-intensity regulator, thereby controlling thewavelength of the third harmonic.
 4. The apparatus for generating awavelength-tunable short pulse according to claim 3, wherein thewavelength of the pulsed light is altered by changing the length of theoptical fiber, thereby controlling the wavelength of the third harmonic.5. The apparatus for generating a wavelength-tunable short pulseaccording to claim 1 or claim 2, wherein the wavelength of the pulsedlight is altered by changing the length of the optical fiber, therebycontrolling the wavelength of the third harmonic.
 6. A method forgenerating a wavelength-tunable short pulse, comprising the steps of:(a) receiving an output from an ultra-short optical pulse source at anoptical fiber, the output having passed through an optical-propertyregulator; (b) generating wavelength-tunable ultrashort pulsed light bya nonlinear optical effect through the soliton effect and Ramanscattering in the optical fiber; and (c) generating a third harmonic ofthe wavelength-tunable ultrashort pulsed light by a third-ordernonlinear optical effect in the optical fiber.
 7. The method forgenerating a wavelength-tunable short pulse according to claim 6,wherein the optical-property regulator is a light-intensity regulator.8. The method for generating a wavelength-tunable short pulse accordingto claim 6 or claim 7, wherein the wavelength of the pulsed light isaltered by changing the intensity of light input to the optical fiber bythe light-intensity regulator, thereby controlling the wavelength of thethird harmonic.
 9. The method for generating a wavelength-tunable shortpulse according to claim 8, wherein the wavelength of the pulsed lightis altered by changing the length of the optical fiber, therebycontrolling the wavelength of the third harmonic.
 10. The method forgenerating a wavelength-tunable short pulse according to claim 6 orclaim 7, wherein the wavelength of the pulsed light is altered bychanging the length of the optical fiber, thereby controlling thewavelength of the third harmonic.